Naomi Osaka showed no signs of discomfort from the injury that forced her to miss a final on Saturday but the former champion still had to dig deep to beat fellow Japanese Misaki Doi 6-2 5-7 6-2 to kick off her US Open campaign.
A left hamstring injury forced Osaka to withdraw from the Western & Southern Open final against Victoria Azarenka but the fourth seed's movement did not seem to be affected against Doi on Monday.
Osaka, the 2018 champion at Flushing Meadows, started strongly in the opening set but struggled in the second as her unforced errors mounted and Doi levelled the match at 1-1 with a second break of her opponent's serve.
Normal service was resumed in the decider for Osaka, who walked out to court wearing a mask featuring the name of Breonna Taylor, a Black woman killed in March by police officers who burst into her apartment.
Osaka will next play Camila Giorgi, who earlier beat Alison van Uytvanck 2-6 6-1 7-5.